This invention relates to the ceramic arc tubes for use in high pressure metal vapor discharge lamps. More particularly, this invention describes a method for forming green ceramic structures which on subsequent sintering result in alumina arc tubes of complex geometries.
Metal vapor discharge lamps consist of an arc tube which holds metal vapor and a protective envelope surrounding the arc tube. The arc tube is required to have both a good translucency of light and a high corrosion resistivity against the light emitting material sealed within, such as sodium vapor or metal halide vapor. Presently, only alumina ceramics have been found to meet the need of high corrosion resistivity against the light emitting material and good translucency, so that alumina ceramics have been used almost exclusively for the arc tubes of high pressure metal vapor discharge lamps.
Typically, enclosed alumina arc tubes are manufactured for use in high pressure metal vapor discharge lamps. Alumina ceramics are used for arc tubes because they exhibit good dimensional stability at elevated temperatures and pressures, high thermal shock resistance and chemical endurance on repeated thermal cycling and high in-line and integrated transmission. Methods for producing these green alumina ceramic arc tubes include isostatic pressing either dry bag or wet bag, slip casting and the use of complex multi piece molds and inflation equipment.
The wet bag isostatic pressing technique for forming arc tubes consists of loading particulate materials of alumina, sintering agents and binding agents and unloading the plastic components outside the pressure vessel This process is labor intensive and requires considerable machining. In an example of one wet isostatic pressing technique a perforated rigid metal cylinder is lined with thin flexible rubber. The perforations allow the pressure transmitting fluid to come in contact with the thin rubber lining. A steel mandrel is located centrally in a rubber bung and the thin, flexible rubber lining is pulled over rubber bung and tied to prevent fluid in the pressure chamber from contacting the particulate aluminum materials. The other end of the perforated metal tube is also closed with a rubber bung and tied with the flexible rubber lining. The particulate material is then consolidated by compacting at pressures ranging from 60 to 100 MPa. The compacted alumina tube is removed carefully from the mandrel, prefired in air for several hours to improve the strength in handling of the green component for subsequent machining operation. Some form of vibratory motion during packing is generally necessary to achieve the uniform packing density and to avoid the swelling effect often seen on translucent tubes after sintering. The swelling effect is usually associated with poor packing of the alumina particulate matter.
In dry bag isostatic pressing, the tooling system is an integral part of the pressure vessel. Filling and removal of the particulate alumina mixture is accomplished mold removal. The technique is more suited for fast production. The dry bag tooling derives its name from the absence of contact of a flexible mold with the pressure transmitting fluid. The latter contacts only the thin rubber sheath lined externally around the flexible mold. The technique of fabricating the thin wall tubes consists of loading particulate alumina and sintering agents into a cavity formed by a mandrel and a mold. Once the particulate matter is loaded into the cavity, pressure is applied to the lined mold which in turn compacts the particulate material. Once the pressure is released the mold is taken apart and the green body of alumina is removed from the mandrel. The green body is ready for prefiring and subsequent sintering.
For the slip casting process, a slip is prepared by combining alumina powder, binders, wetting agents, sintering aids and dispersing agents in a selected casting liquid. The slip is allowed to age until its characteristics are relatively constant. The slip is then poured into a mold that has the capability of absorbing water and causing the wet powder to stick the mold wall. The excess slurry or slip is poured from the center of the mold, leaving a green body within the mold. Upon drying, this body shrinks from the mold wall and can be removed by opening the mold. At present, it is not known whether this method is used for the manufacture of arc tubes.
Another technique for producing ceramic arc tubes is disclosed in U.S. Pat. No. 4,387,067. In this patent a method for producing a green ceramic body is disclosed. The method consists of preparing and molding a mixture of alumina particles and plasticizer. A tubular green body is formed with a molding machine and subsequently placed within a fusiform cavity of a die and fluid pressure is applied to the inside of the tubular green body, so as to shape the tubular green body by inflating the middle portion of the die. The green shaped body is removed from the die and fired and sintered.
The above-identified methods for producing ceramic arc tubes all have serious drawbacks. The isostatic pressure methods are extremely labor intensive and are thus not conducive to large scale production. Furthermore, these methods require complex mandrels and bags. The method described in U.S. Pat. No. 4,387,067 requires complex molds and produces arc tubes wherein the wall thickness is not consistent over the entire length of the tube. The present invention solves these problems and results in a considerable savings of time and labor. In addition, the present invention allows one to create arc tubes of relatively complex shapes without requiring complex molds or mandrels.